The present invention relates to transducers for use in inductive position sensors and to methods for their manufacture. The present invention has particular although not exclusive relevance to the manufacture of conductive tracks for sensing or for generating magnetic fields for use in an inductive position sensor using wire bonding technology.
Many types of non contact linear and rotary position encoders have been proposed for generating signals indicative of the position of two relatively movable members. Typically, one of the members carries one or more sensor coils and the other carries one or more magnetic field generators. The magnetic field generators and the sensor coils are arranged such that the amount of magnetic coupling between them varies as a function of the relative position between the two members. This can be achieved by, for example, designing the sensor coils so that their sensitivity to magnetic field varies in a predetermined manner along the measurement path. Alternatively, the magnetic field generators can be designed so that the magnetic field which they generate varies in a predetermined manner along the measurement path.
One example of this type of position encoder is the Inductosyn, which comprises a contactless slider which is arranged to detect the field generated by a fixed track, or vice versa. The fixed track comprises a repeating pattern of conductors which generate a magnetic field of substantially sinusoidal variation in the measurement direction when a current is applied to them. This magnetic field is detected by the moving slider, which comprises sin and cos detector tracks. The position of the two relatively movable members is then determined from the spatial phase of the signals detected by these two detector tracks.
The applicant has proposed in its earlier International Application WO95/31696, a similar type of position encoder in which one member carries an excitation coil and a number of sensor coils and the other member carries a resonator. In operation, the excitation coil energises the resonator which induces signals in the sensor coils which sinusoidally vary with the relative position between the two members.
In both these prior art systems, in order for the output signals to accurately reflect the relative position between the two members, the sensor coils and/or the magnetic field generators must be accurately positioned on the respective members.
Most commercial systems to date either employ screen printing technology using conductive inks or printed circuit board (PCB) technology, to create the sensor coils and/or the magnetic field generators. However, the screen printing technique suffers from the disadvantage that the tracks produced have a relatively high resistance (as compared with those produced by the PCB technology) resulting in either low output signal levels if the tracks are for sensing magnetic fields, or the necessity of large transmitting powers in order to generate the required strength of magnetic field if the tracks are for generating magnetic fields. Although the tracks produced using the PCB technology have a lower resistance than those of the screen printed inks, PCB technology suffers from a number of disadvantages, including:
i) existing PCB processing techniques are predominantly batch based with maximum board dimensions of approximately 0.6 m;
ii) existing PCB techniques typically employ multiple layers with through connections (vias) which are difficult to manufacture, especially with long tracks, because their manufacture typically requires stationary batch-based processing;
iii) positional errors are generated in the output signals, because the conductors do not lie on a single layer but on two or more separate layers.
U.S. Pat. No. 5,625,239 discloses a position encoder for use in determining the relative position between a rotor and a stator, wherein the rotor carries a transmitter coil made from wire which is laid in a groove and then secured by means of an adhesive. However, this technique requires the manufacture of precise grooves around the rotor and is therefore expensive to make.
One aim of the present invention is to provide an alternative method of manufacturing sensor coils and/or magnetic field generators for use in position sensors.
According to one aspect, the present invention provides a method of manufacturing a transducer for use in sensing or generating magnetic fields which employs wire bonding technology to lay the conductor tracks on a substrate in the required shape. This has the benefits of low cost, accuracy and simplicity and the potential for easily making long lengths or awkward shapes of transducer.
According to another aspect, the present invention provides a method of manufacturing a transducer for use in a position encoder, the method comprising the steps of: providing a wire laying device for controllably laying a wire onto a substrate; causing the laying device to lay the wire on the substrate in a desired pattern by causing the relative movement between the laying device and the substrate; and bonding the wire to the substrate. Preferably, the wire is bonded to the substrate whilst it is being laid, since this improves the laying accuracy.
The method described above is ideally suited to manufacturing long (typically  greater than 0.6 m) lengths of transducer because the substrate can be continuously moved or stepped past the laying device. In contrast, with the prior art techniques, in order to make long length tracks, they are either made in sections and then subsequently joined together or they are stretched to the required length. However, both techniques are expensive, time consuming and require significant skill to align the sections properly or to ensure the stretched track has the required shape.
Preferably the substrate comprises a rigid steel support for providing mechanical stability to the substrate. In this case, in order to allow a low profile transducer to be manufactured, a layer of magnetically soft material is preferably disposed between the steel base layer and the wires, since such a magnetically soft layer enhances performance by providing a permeable path for magnetic flux to pass behind the wires. The magnetically soft layer may comprise a flexible plastic or rubber layer containing ferrite or iron powder.
Where the scale is required in lengths in excess of 2 m, the scale is preferably flexible so that it can be coiled up for ease of transportation and storage. In this case, the substrate preferably comprises a flexible yet stable support such as spring steel which has substantial linear stability when made flat after being coiled up.
Although the wires can be glued to the substrate, ultrasonic or heat bonding is preferably used since the position can be fixed almost instantaneously, so that the placement of the wire can occur at the same time as its position is fixed.
Preferably the wire bonding machine comprises a sensor for detecting the height of the wire bonding head relative to the substrate in order to control the bonding pressure and to be able to accommodate different substrate thicknesses. Preferably, the wire bonding machine also comprises means for terminating the wires (for example by welding them onto terminals) and lamination means for laminating a protective upper layer over the wires for protection and so that further layers of wires can be laminated on top, for example, so that the transducer can be used in an absolute position encoder.
According to another aspect, the present invention provides a method of manufacturing a transducer for use in a position detector, the method comprising the steps of providing a wire laying device for controllably laying a wire onto a substrate causing the laying device to lay the wire on the substrate in a desired pattern by (i) sensing the current position of the wire laying device relative to the substrate; (ii) comparing said current position with an expected position derived from said desired pattern; and (iii) controlling the relative movement between said wire laying device and said substrate in dependence upon the result of said comparison step and bonding the wire to the substrate to form the desired pattern.
The present invention also provides a position detector comprising first and second members mounted for relative movement along a measuring path said first member, comprising a magnetic field generator for generating a magnetic field along said measuring path said second member comprising a magnetic field sensor for sensing the magnetic field generated by said magnetic field generator and for outputting an output signal, said magnetic field sensor and said magnetic field generator being arranged so that said output signal varies as a function of the relative position of said two members along said path characterised in that at least one of said magnetic field generator and said magnetic field sensor comprises a transducer manufactured using the above described method.
The present invention also provides a transducer for use in sensing or generating magnetic fields in a position detector, the transducer comprising a substrate and at least one wire bonded onto the substrate in a desired pattern using the above described method.
According to another aspect, the present invention provides a transducer for use in a position detector, the transducer comprising a substrate; a first layer of wire bonded onto the substrate in a desired pattern; an insulating layer positioned on said first layer of wire; and a second layer of wire bonded onto said insulating layer in a desired pattern.
According to another aspect, the present invention provides a wire bonding apparatus for manufacturing a transducer for use in sensing or generating magnetic fields in a position detector, the wire bonding apparatus comprising a wire laying device for controllably laying a wire onto a substrate; means for sensing the current position of the wire laying device relative to the substrate; means for comparing the current position with an expected position derived from said desired pattern; controlling means for controlling the relative movement between said wire laying device and said substrate in dependence upon the output of said comparison means; and bonding means for bonding the wire to the substrate in the desired pattern.
According to another aspect, the present invention provides a transducer for use in sensing or generating magnetic fields in a position sensor, the transducer comprising a layer of at least one conductor for sensing or for generating magnetic fields; and a layer of soft magnetic material located behind said conductor layer for providing a permeable path for magnetic flux to pass behind the conductor layer.